Fuel cell vehicles, which are fueled by hydrogen and oxygen, obtain electric power without exhausting hazardous substances such as carbon dioxide (CO2), nitrogen oxide (NOx), and sulfur oxide (SOx), and thus have attracted attention as clean vehicles of the next generation in place of gasoline vehicles and diesel engine vehicles. In Japan, commercial sale of fuel cell vehicles carrying hydrogen gas cylinders started in 2002, and the fuel cell vehicles for sale are increasing in number year by year. However, the current fuel cell vehicles can travel only as far as 300 km due to dimensional restrictions of the cylinders, and this has posed an obstacle to the wide use of these vehicles. In order to improve the travel distance, it is effective to make the pressure of the hydrogen gas accommodated in on-board cylinders, as high as 35 to 70 MPa, and thus various kinds of equipment associated with hydrogen gas, such as storage containers, pipes, and injection valves, need to use safe materials in the high-pressure hydrogen environment.
However, the use of steel materials in a hydrogen gas environment, particularly a high-pressure hydrogen gas environment, poses the problem of hydrogen embrittlement caused by the hydrogen gas. This phenomenon is called “Hydrogen Environment Embrittlement (HEE),” which is known as a phenomenon which degrades the mechanical properties of metallic materials, such as ductility and rupture stress in hydrogen gas environment. In Japan, various material evaluations have been carried out since research on the basic physical properties of material for hydrogen started in 2003 in “basic technical development on safe utilization of hydrogen” of New Energy and Industrial Technology Development Organization (NEDO).
This resulted in papers such as non-patent document 1. Non-patent document 1 mentions an aluminum alloy A6061-T6 and also a stable austenitic stainless steel SUS316L, as examples of the metallic material hardly embrittled in a high-pressure hydrogen gas environment. These metallic materials have a fcc (face-centered cubic) structure, which is generally believed to hardly involve hydrogen embrittlement. The result of the research is believed to serve as a basis for High Pressure Gas Safety Law, Section 3 (material) of the Standardized Examples for Hydrogen Fuel Containers for Compressed Hydrogen-fuel Vehicles. However, with the A6161-T6 having a tensile strength of no more than approximately 300 MPa, and with the SUS type austenitic stainless steel having a tensile strength of no more than approximately 500 to 600 MPa, they cannot sufficiently meet the demand for even higher strength in order to reduce the weight of on-board containers.
High-strength low alloy steels are tempting for the above demand because they have high strength and realize reduction in production cost, but they have a bcc (body-centered cubic) structure, which is believed to be highly susceptible to hydrogen embrittlement; in particular, the embrittlement susceptibility is known to increase as the strength increases. Among very few evaluations ever conducted for detailed properties of low alloy steel in a high-pressure hydrogen environment, non-patent document 2 reports an experiment using low alloy steels (AISI4340 steel, 4130 steel, and high manganese steel) as specimens, where the embrittlement susceptibility increases when the tensile strength exceeds 900 MPa. Thus, although some of the low alloy steel containers are being applied to pressure accumulators at hydrogen stations on the condition that regular maintenance is conducted. It is generally believed difficult to apply low alloy steels to on-board containers, because they are difficult to regularly maintain.
In patent document 1, the applicant proposes an invention related to a steel product for a cylinder, consisting of, by mass percent, C, 0.20 to 0.35%, Si: ≦0.35, Mn: 0.3 to 2.0%, P: ≦0.025%, S: ≦0.015%, Cr: 0.8 to 2.0%, Mo: 0.3 to 1.0%, B: 0.0005 to 0.0030%, Al: 0.01 to 0.10%, and N: ≦0.008%, or further at least one selected from Nb: ≦0.10%, Ti: ≦0.10%, Cu: ≦2.00%, Ni: ≦2.00%, V: ≦0.10%, and Ca: ≦0.010%, and the balance Fe and impurities.    [Patent document 1] Japanese Patent Laid-open No. 2005-139499    [Non-patent document 1] Tamura Motonori et al., “Evaluation of Mechanical Properties of Metals at 45 MPa Hydrogen”, J. Japan Inst. Metals, vol. 69, No. 12 (2005), 1039-1048.    [Non-patent document 2] Hinotani Shigeharu et al., “Hydrogen Embrittlement of High Strength Steels in High Pressure Hydrogen Gas at Ambient Temperature”, Tetsu-to-Hagane, 64th year, No. 7 (1978), 899-905.